1. Field of the Invention
The present invention relates generally to photomultiplier tubes and, more particularly, to an automatic gain stabilization system for use with them. The method utilizes a light source, preferably a light emitting diode (LED), the signal of which is detected both at the first dynode and at the anode. By keeping the ratio between these two signals constant the actual gain of the photomultiplier tube is stabilized and possible drifts in the intensity of the light source are eliminated.
2. Description of the Prior Art
Photomultiplier tubes, or shortly photomultipliers, are common instruments in science and technology for detecting weak light levels. The photomultiplier consists of a photosensitive cathode, a chain of secondary emission electrodes called dynodes and an output electrode called anode with electric potentials arranged between them. The operation principle is as follows: Light flux hits the cathode which converts light photons into free electrons. The applied voltage directs them to the first dynode, from which every electron liberates several secondary electrons in a process called secondary emission. These are in turn directed to the next dynode, where the secondary emission is repeated and so on. The result is amplification by electron multiplication so that after the dynode chain the signal taken from the anode is high enough to be handled electronically. In some applications one of the later dynodes can be used as an output electrode.
An important quantity associated with photomultipliers is their amplification, or gain, defined as the ratio of anode current to cathode current and is typically 10.sup.5 -10.sup.9 depending on the number of dynodes, interdynode voltages and dynode materials. The gain should, naturally, remain stable during operation to yield ideal performance for the light detecting device. Unfortunately, this is not normally achieved but the gain tends to drift with temperature, variable light fluxes and ageing of the photomultiplier.
For correcting the gain instabilities a known solution is to employ a supplementary pulsed light source with standardized intensity to monitor the output of the photomultiplier and to adjust the gain according to the obtained signal by e.g. a feedback loop as presented by Ried and Gilland (U.S. Pat. No. 3,515,878). The pulsed light source can be e.g. a low-intensity lamp, a light emitting diode (LED) or a radioactive isotope in conjunction with appropriate scintillator.
A problem with mentioned stabilization light sources is that they, too, are susceptible to instabilities. These can be caused by thermal drifts, ageing, and alterations in reflective and/or absorptive properties in materials surrounding the light source-photomultiplier assembly. Consequently, the photomultiplier gain can never be more stable than the used stabilization light source. Accordingly, there is a need for a gain stabilization method that is not sensitive to drifts in the stabilization light sources. The present invention meets this requirement.